checking if a list is empty

[Steven D'Aprano]

On Mon, 09 May 2011 16:58:14 -0500, harrismh777 wrote:

> Ian Kelly wrote:
>> "bool(list)" describes whether the list contains something.  "Not"
>> being a logical operator, it stands to reason that "not list" should
>> mean the same thing as "not bool(list)".
> 
> Ian, James,
> 
>     Agreed, and thank you.  This *is* the explanation I was trying to
> prompt D'Aprano for, rather than getting his 'not cat' analogy.

Your dislike of analogies is leading you to see them where they aren't. 
The "cat on the mat" sentence I gave is a concrete example of the use of 
negation in English, not an analogy. It is virtually the opposite of an 
analogy.

In that same post that annoyed you with the cat on the mat example, I 
wrote:

    Python uses a boolean algebra where there are many ways of 
    spelling the true and false values. The "not" operator returns 
    the canonical bool values:

    not <any true value> returns False
    not <any false value> returns True

    Take note of the distinction between lower-case true/false, 
    which are adjectives, and True/False, which are objects of 
    class bool.

and later on, I contrasted:

    empty list versus non-empty list

Ergo, "not (empty list)" returns True, and "not (non-empty list)" returns 
False, because the empty list is considered false and any non-empty list 
is considered true. I'm sorry that I failed to make that more explicit. 
If I were talking to a programming n00b, I would have been more careful, 
but you've made numerous references to your long, long programming 
experience and I thought you would be able to draw the obvious connection 
without me insulting you by stating the obvious.



-- 
Steven